Microspheres

ABSTRACT

The invention relates to a process for the production of paper or nonwoven fibres comprising the steps of adding thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein to a stock comprising fibres or to a web of fibres, forming paper or nonwoven from the stock or the web, and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk of the paper or the nonwoven, wherein said expandable microspheres comprise from about 17 to about 40 wt % propellant and have a volume-average diameter from about 17 to about 35 &amp;micro;m. The invention also concerns certain expandable microspheres and use thereof.

The present invention relates to a process for the production of paper or nonwoven and thermoplastic expandable microspheres useful therefore.

Expandable thermoplastic microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein are commercially available under the trademark EXPANCEL® and are used as a foaming agent in many different applications.

In such microspheres, the propellant is normally a liquid having a boiling temperature not higher than the softening temperature of the thermoplastic polymer shell. Upon heating, the propellant evaporates to increase the internal pressure at the same time as the shell softens, resulting in significant expansion of the microspheres. The temperature at which the expansion starts is called T_(start), while the temperature at which maximum expansion is reached is called T_(max). Expandable microspheres are marketed in various forms, e.g. as dry free flowing particles, as an aqueous slurry or as a partially dewatered wet-cake.

Expandable microspheres can be produced by polymerising ethylenically unsaturated monomers in the presence of a propellant. Detailed descriptions of various expandable microspheres and their production can be found in, for example, U.S. Pat. Nos. 3,615,972, 3,945,956, 5,536,756, 6,235,800, 6,235,394 and 6,509,384, and in EP 486080.

It has been disclosed to use microspheres in papermaking, for example in U.S. Pat. Nos. 3,556,934 and 4,133,688, JP Patent 2689787 and in Ö. Söderberg, “World Pulp & Paper Technology 1995/96, The International Review for the Pulp & Paper Industry” p. 143-145.

It is an object of the invention to provide a process for the production of paper or nonwoven with low bulk density.

It is another object of the invention to provide expandable thermoplastic microspheres that can be used in the production of paper or nonwoven with low bulk density.

It has previously been believed that expandable thermoplastic microspheres of large size would have poor expansion properties. However, it has now been found that such microspheres, when also having high content of propellant, give higher expansion than expected when used in the production of paper or nonwoven for increasing the bulk thereof.

The invention thus concerns use of thermally expandable microspheres comprising a thermoplastic polymer shell and from about 17 to about 40 wt %, preferably from about 18 to about 40 wt %, most preferably from about 19 to about 40 wt %, particularly most preferably from about 20 to about 35 wt % of a propellant entrapped in said polymer shell, and having a volume-average diameter from about 17 to about 35 μm, preferably from about 18 to about 35 μm, more preferably from about 19 to about 35 μm, most preferably from about 20 to about 30 μm, particularly most preferably from about 21 to about 30 μm, in the production of paper or non-woven for Increasing the bulk thereof.

The term expandable microspheres as used herein refers to expandable microspheres that have not previously been expanded, i.e. unexpanded expandable microspheres.

All figures for volume-average diameter given herein refer to values obtained by measuring according to ISO 13319:2000, “Determination of particle size distributions—Electrical sensing zone method”. Detailed description of this measuring method can be obtained from, for example, Swedish Institute For Standards, Stockholm.

The invention further concerns a process for the production of paper or nonwoven from fibres comprising the steps of adding thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein to a stock comprising fibres or to a web of fibres, forming paper or nonwoven from the stock or the web, and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk of the paper or the nonwoven. The expandable microspheres have a volume-average diameter from about 17 to about 35 μm, preferably from about 18 to about 35 μm, more preferably from about 19 to about 35 μm, most preferably from about 20 to about 30 μm, particularly most preferably from about 21 to about 30 μm. The amount of propellant in the expandable microspheres is from about 17 to about 40 wt %, preferably from about 18 to about 40 wt %, most preferably from about 19 to about 40 wt %, particularly most preferably from about 20 to about 35 wt %.

An embodiment of the invention concerns a process for the production of paper comprising the steps of adding expandable microspheres as described above to a stock containing cellulosic fibres, dewatering the stock on a wire to obtain paper, and drying the paper by applying heat and thereby also raising the temperature of the microspheres sufficiently for them to expand and increase the bulk of the paper. The expandable microspheres may be added separately or together with one or more other additive used in the papermaking process.

The expandable microspheres can be added in any form, although it from a practical point of view is most preferred to add them in the form of an aqueous slurry, preferably having a solids content from about 5 to about 55 wt %, most preferably from about 40 to about 50 wt %. The slurry preferably also comprises a thickener compatible with paper making, such as anionic or cationic starch, optionally in combination with a salt such as sodium chloride. Starch may, for example, be present in the slurry in an amount from about 0.1 to about 5 wt %, preferably from about 0.3 to about 1.5 wt %. Sodium chloride, or another salt, may, for example, be present in the slurry in an amount from about 0.1 to about 20 wt %, preferably from about 1 to about 15 wt %.

The amount of expandable microspheres added to the stock is preferably from about 0.1 to about 20 wt %, most preferably from about 0.2 to about 10 wt % dry microspheres of the dry content in the stock. Any kind of paper machine known in the art can be used.

The term “paper”, as used herein, is meant to include all types of cellulose-based products in sheet or web form, including, for example, board, cardboard and paperboard. The invention has been found particularly advantageous for the production of board, cardboard and paper board, particularly with a basis weight from about 50 to about 1000 g/m², preferably from about 150 to about 800 g/m².

The paper may be produced as a single layer or a multi-layer paper. If the paper comprises three or more layers, the expandable microspheres are preferably not added to the portion of the stock forming any of the two outer layers.

The stock preferably contains from about 50 to about 100 wt %, most preferably from about 70 to about 100 wt % of cellulosic fibres, based on dry material. Before dewatering, the stock besides expandable microspheres, may also contain one or more fillers, e.g. mineral fillers like kaolin, china clay, titanium dioxide, gypsum, talc, chalk, ground marble or precipitated calcium carbonate, and optionally other commonly used additives, such as retention aids, sizing agents, aluminium compounds, dyes, wet-strength resins, optical brightening agents, etc. Examples of aluminium compounds include alum, aluminates and polyaluminium compounds, e.g. polyaluminium chlorides and sulphates. Examples of retention aids include cationic polymers, anionic inorganic materials in combination with organic polymers, e.g. bentonite in combination with cationic polymers or silica-based sols in combination with cationic polymers or cationic and anionic polymers. Examples of sizing agents include cellulose reactive sizes such as alkyl ketene dimers and alkenyl succinic anhydride, and cellulose non-reactive sizes such as rosin, starch and other polymeric sizes like copolymers of styrene with vinyl monomers such as maleic anhydride, acrylic acid and its alkyl esters, acrylamide, etc.

At drying, the paper, and thereby also the microspheres, is preferably heated to a temperature from about 50 to about 150° C., most preferably from about 60 to about 110° C. This results in expansion of the microspheres and thereby also a bulk increase of the paper. The magnitude of this bulk increase depends on various factors, such as the origin of cellulosic fibres and other components in the stock, but is in most cases from about 5 to about 50% per weight percentage of retained microspheres in the dried paper, compared to the same kind of paper produced without addition of expandable microspheres or any other expansion agent. Any conventional means of drying involving transferring heat to the paper can be applied, such as contact drying (e.g. by heated cylinders), forced convection drying (e.g. by hot air), infrared techniques, or combinations thereof. In the case of contact drying, the temperature of the contact surfaces, e.g. the cylinders, is preferably from about 20 to about 150° C., most preferably from about 30 to about 130° C. The paper may pass a series of several cylinders, e.g. up to 20 or more, of increasing temperature.

The cellulosic fibres in the stock may, for example, come from pulp made from any kind of plants, preferably wood, such as hardwood and softwood. The cellulosic fibres may also partly or fully originate from recycled paper, in which case the invention has been found to give unexpectedly good results.

Another embodiment of the invention concerns a process for the production of nonwoven comprising the steps of forming a web of fibres, adding to said web a binder and expandable microspheres as described above, and forming nonwoven and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk nonwoven. The expandable microspheres and the binder may be added separately or as a mixture. The amount of expandable microspheres added is preferably from about 0.1 to about 30 wt % of dried product, most preferably from about 0.5 to about 15 wt % of dried product. The amount of binder added is preferably from about 10 to about 90 wt % of dried product, most preferably from about 20 to about 80 wt % of dried product

The term “nonwoven” as used herein is meant to include textiles made from fibres bonded together by means of a binder.

The web of fibres can be formed in any conventional way, for example by mechanical or aerodynamical dry methods, hydrodynamical (wet) methods, or spunbonded processes. The binder, preferably pre-mixed with expandable microspheres, can then be added to the web also in any conventional way, for example by any kind of impregnation method such as immersion of the web in a bath of binder or coating the web by kiss roll application or knife coating with a doctor blade or floating knife.

The web comprising a binder and expandable microspheres can then be heated to a temperature sufficient for the microspheres to expand, preferably from about 70 to about 200° C., most preferably from about 120 to about i60° C. Preferably, curing of the binder takes place at the same time. The heating can be effected by any suitable means, such as contact drying (e.g. by heated cylinders), forced convection drying (e.g. by hot air), infrared techniques, or combinations thereof.

The fibres can be any kind of commercially available fibres, natural fibres, mineral fibres, as well as synthetic inorganic and organic fibres. Examples of useful fibres include polypropylene, polyethylene, polyester, viscose, and polyamide fibres, as well as fibres made from two or more of the above polymers.

The binder can be any kind of natural or synthetic adhesive resin, such as resins of polyacrylates and co-polymers thereof, polymethacrylates and co-polymers thereof, rubber latexes such as styrene/butadiene copolymers, acrylonitrilelbutadiene copolymers, poly(vinyl chloride) and copolymers, poly(vinyl ester) such as poly(vinyl acetate) and copolymers, e.g. with ethylene, poly(vinyl alcohol), polyurethane, and aminoplast and phenoplast precondensates such as urea/formaldehyde, urea/melamine/formaldehyde or phenol/formaldehyde.

Preferred expandable microspheres to be used according to the invention are described below.

The thermoplastic polymer shell of the expandable microspheres is suitably made of a homo- or co-polymer obtained by polymerising ethylenically unsaturated monomers. Those monomers can, for example, be nitrile containing monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, fumaronitrile or crotonitrile; acrylic esters such as methyl acrylate or ethyl acrylate; methacrylic esters such as methyl methacrylate, isobomyl methacrylate or ethyl methacrylate; vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate other vinyl monomers such as vinyl pyridine; vinylidene halides such as vinylidene chloride; styrenes such as styrene, halogenated styrenes or α-methyl styrene; or dienes such as butadiene, isoprene and chloroprene. Any mixtures of the above mentioned monomers may also be used.

Preferably the monomers comprise at least one acrylic ester or methacrylic ester monomer, most preferably methacrylic ester monomer such as methyl methacrylate. The amount thereof in the polymer shell is preferably from about 0.1 to about 80 wt %, most preferably from about 1 to about 25 wt % of the total amounts of monomers.

Preferably the monomers comprise at least one vinylidene halide monomer, most preferably vinylidene chloride. The amount thereof in the polymer shell is preferably from about 1 to about 90 wt %, most preferably from about 20 to about 80 wt % of the total amounts of monomers.

Most preferably the monomers comprise both at least one acrylic ester or methacrylic ester monomer and at least one vinylidene halide monomer.

Preferably the monomers comprise at least one nitrile containing monomer, most preferably at least one of acrylonitrile and methacrylonitrile, particularly most preferably at least acrylonitrile. The amount thereof in the polymer shell is preferably from about 1 to about 80 wt %, most preferably from about 20 wt % to about 70 wt % of the total amounts of monomers.

In an advantageous embodiment the monomers comprise at least one acrylic ester monomer, at least one vinylidene halide and at least one nitrile containing monomer. The polymer shell may then, for example, be a co-polymer obtained from monomers comprising methyl methacrylate in a preferred amount from about 0.1 to about 80 wt %, most preferably from about 1 to about 25 wt % of the total amounts of monomers, vinylidene chloride in a preferred amount from about 1 to about 90 wt %, most preferably from about 20 to about 80 wt % of the total amounts of monomers, and acrylontrile in a preferred amount from about 1 to about 80 wt %, most preferably from about 20 to about 70 wt % of the total amounts of monomers.

It may sometimes be desirable that the monomers for the polymer shell also comprise crosslinking multifunctional monomers, such as at least one of divinyl benzene, ethylene glycol di(meth)acrylate, di(ethylene glycol) di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glycerol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, triallylformal tri(meth)acrylate, allyl methacrylate, trimethylol propane tri(meth)acrylate, tributanediol di(meth)acrylate, PEG #200 di(meth)acrylate, PEG #400 di(meth)acrylate, PEG #600 di(meth)acrylate, 3-acryloyloxyglycol monoacrylate, triacryl formal or triallyl isocyanate, triallyl isocyanurate etc. The amount thereof in the polymer shell is preferably from about 0.1 to about 10 wt %, most preferably from about 0.1 to about 1 wt %, particularly most preferably from about 0.2 to about 0.5 wt % of the total amounts of monomers.

The propellant is normally a liquid having a boiling temperature not higher than the softening temperature of the thermoplastic polymer shell and may comprise hydrocarbons such as propane, n-pentane, isopentane, neopentane, butane, isobutane, hexane, isohexane, neohexane, heptane, isoheptane, octane or isooctane, or mixtures thereof. Aside from them, other hydrocarbon types can also be used, such as petroleum ether, or chlorinated or fluorinated hydrocarbons, such as methyl chloride, methylene chloride, dichloroethane, dichloroethylene, trichloroethane, trichloroethylene, trichlorofluoromethane, perfluorinated hydrocarbons, etc. Preferred propellants comprise isobutane, alone or in a mixture with one or more other hydrocarbons. The boiling point at atmospheric pressure is preferably within the range from about −50 to about 100° C., most preferably from about −20 to about 50° C., particularly most preferably from about −20 to about 30° C.

Apart from the polymer shell and the propellant the microspheres may comprise further substances added during the production thereof, normally in an amount from about 1 to about 20 wt %, preferably from about 2 to about 10 wt %. Examples of such substances are solid suspending agents, such as one or more of silica, chalk, bentonite, starch, crosslinked polymers, methyl cellulose, gum agar, hydroxypropyl methylcellulose, carboxy methylcellulose, colloidal clays, and/or one or more salts, oxides or hydroxides of metals like Al, Ca, Mg, Ba, Fe, Zn, Ni and Mn, for example one or more of calcium phosphate, calcium carbonate, magnesium hydroxide, barium sulphate, calcium oxalate, and hydroxides of aluminium, iron, zinc, nickel or manganese. If present, these solid suspending agents are normally mainly located to the outer surface of the polymer shell. However, even if a suspending agent has been added during the production of the microspheres, this may have been washed off at a later stage and could thus be substantially absent from the final product

Some of the microspheres described above are novel. The invention thus also concerns thermally expandable microspheres comprising a thermoplastic polymer shell made of a co-polymer obtained by polymerising ethylenically unsaturated monomers comprising at least one acrylic ester or methacrylic ester monomer and at least one vinylidene halide monomer, and from about 17 to about 40 wt %, preferably from about 18 to about 40 wt %, most preferably from about 19 to about 40 wt %, particularly most preferably from about 20 to about 35 wt % of a propellant entrapped in said polymer shell, wherein the expandable microspheres have a volume-average diameter from about 17 to about 35 μm, preferably from about 18 to about 35 μm, more preferably from about 19 to about 35 μm, most preferably from about 20 to about 30 μm, particularly most preferably from about 21 to about 30 μm. Regarding further possible and preferred embodiments of the novel microspheres, applicable parts of the above description of the process for the production of paper or nonwoven is referred to.

The novel expandable microspheres can be prepared by polymerising the monomers in the presence of the propellant with the same methods as described in the earlier mentioned U.S. Pat. Nos. 3,615,972, 3,945,956, 5,536,756, 6,235,800, 6,235,394 and 6,509,384, and in EP 486080.

In a preferred batchwise procedure for producing expandable microspheres, the polymerisation is conducted as described below in a reaction vessel. For 100 parts of monomer phase (suitably including monomers and propellant, the ratio of which determines the amount of propellant in the final product), one or more polymerisation initiator, preferably in an amount from about 0.1 to about 5 parts, aqueous phase, preferably in an amount from about 100 to about 800 parts, and one or more preferably solid colloidal suspending agent, preferably in an amount from about 1 to about 20 parts, are mixed and homogenised. The size of the droplets of monomer phase obtained determines the size of the final expandable microspheres, in accordance with principles described in e.g. U.S. Pat. No. 3,615,972 and can be applied for all similar production methods with various suspending agents. The temperature is suitably maintained from about 40 to about 90° C., preferably from about 50 to about 80° C., while the suitable pH depends on the suspending agent used. For example, a high pH, preferably from about 6 to about 12, most preferably from about 8 to about 10, is suitable if the suspending agent is selected from salts, oxides or hydroxides of metals like Al, Ca, Mg, Ba, Fe, Zn, Ni and Mn, for example one or more of calcium phosphate, calcium carbonate, chalk, magnesium hydroxide, barium sulphate, calcium oxalate, and hydroxides of aluminium, iron, zinc, nickel or manganese. A low pH, preferably from about 1 to about 6, most preferably from about 3 to about 5, is suitable if the suspending agent is selected from silica, bentonite, starch, methyl cellulose, gum agar, hydroxypropyl methylcellulose, carboxy methylcellulose, colloidal clays. Each one of the above agents have different optimal pH, depending on, for example, solubility data.

In order to enhance the effect of the suspending agent, it is also possible to add small amounts of one or more promoters, for example from about 0.001 to about 1 wt %. Usually, such promoters are organic materials and may, for example, be selected from one or more of water-soluble sulfonated polystyrenes, alginates, carboxymethylcellulose, tetramethyl ammonium hydroxide or chloride or water-soluble complex resinous amine condensaton products such as the water-soluble condensation products of diethanolamine and adipic acid, the water-soluble condensation products of ethylene oxide, urea and formaldehyde, polyethylenimine, polyvinylalcohol, polyvinylpyrrolidone, amphoteric materials such as proteinaceous, materials like gelatin, glue, casein, albumin, glutin and the like, non-ionic materials like methoxycellulose, ionic materials normally classed as emulsifiers, such as soaps, alkyl sulfates and sulfonates and long chain quaternary ammonium compounds.

Conventional radical polymerisation may be used and initiators are suitably selected from one or more of organic peroxides such as dialkyl peroxides, diacyl peroxides, peroxy esters, peroxy dlcarbonates, or azo compounds. Suitable initiators include dicetyl peroxy dicarbonate, tert-butyl cyclohexyl peroxy dicarbonate, dioctanoyl peroxide, dibenzoyl peroxide, dilauroyl peroxide, didecanoyl peroxide, tert-butyl peracetate, tert-butyl perlaurate, tert-butyl perbenzoate, tert-butyl hydroperoxide, cumene hydroperoxide, cumene ethylperoxide, dilsopropyl hydroxy dicarboxylate, azo-bis dimethyl valeronitrile, azo-bis isobutyronitrile, azo-bis (cyclo hexyl carbonitrile) and the like. It is also possible to initiate the polymerisation with radiation, such as high energy ionising radiation.

When the polymerisation is essentially complete, microspheres are normally obtained as an aqueous slurry or dispersion,. which can be dewatered by any conventional means, such as bed filtering, filter pressing, leaf filtering, rotary filtering, belt filtering or centrifuging to obtain a so called wet cake that can be used as such. However, it is also possible to dry the microspheres by any conventional means, such as spray drying, shelf drying, tunnel drying, rotary drying, drum drying, pneumatic drying, turbo shelf drying, disc drying or fluidised bed-drying.

The invention will now be further described in connections with the following Examples, which, however, not should be interpreted as limiting the scope thereof. If not otherwise stated, all parts and percentages refer to parts and percent by weight.

EXAMPLE 1

A three layer paper board with a basis weight of about 180 g/m² was produced in a pilot paper machine with a machine speed of 7 m/min and having recirculated process water. The pulp was composed of 40 wt % hardwood and 60 wt % softwood pulp and was beaten to a Schopper-Riegler value of 25° SR and then dispersed to give a pulp slurry/stock. An aqueous slurry of expandable microspheres was before the mixing pump added to the stock used for the middle layer in an amount of about 1 wt % dry microspheres of the dry substance in the stock. As retention aid 0.1 wt % Polymin™ SK was used. In the drying section the paper web was heated by cylinders having a temperature profile from 30 to 130° C. Different kinds of expandable microspheres were tested, all having isobutane as propellant and a polymer shell from vinylidene chloride (VDC), acrylonitrile (ACN) and methyl methacrylate (MMA) but In various ratios. In order to determine the retention of the microspheres, paper samples were taken before the press section for determination of the amount of microspheres (using GC). The retention was calculated from the microspheres addition and the content of microspheres in the paper. Moreover, samples from the dried paper were taken for determination of bulk and thickness. The results are shown in Table 1. TABLE 1 VDC/ACN/MMA in Amount of Increased bulk polymer shell propellant Particle size (% per percentage of (wt %) (wt %) (μm) retained microspheres) 56/35/9 15.8 13.2 6 56/35/9 24.4 24.6 39 73/24/3 16.5 12.3 19 73/24/3 24.8 28.0 39

EXAMPLE 2

A single layer paper board with a basis weight of about 200 g/m² was produced in a pilot paper machine with a machine speed of 4 m/min and not having recirculated process water. The pulp was composed of 50 wt % hardwood and 50 wt % softwood pulp and was beaten to a Schopper-Riegler value of 25° SR and then dispersed to give a pulp slurry/stock. An aqueous slurry of expandable microspheres was before the mixing pump added to the stock in an amount of about 1.75 wt % dry microspheres of the dry substance in the stock. As retention aid Compozil®, 0.1% BMA-O™ and 0.75% Raisamy™ 135, was used. In the drying section the paper web was heated by cylinders having a temperature profile from 65 to 122° C. Expandable microspheres with the same propellant and same monomers in the polymer shell as in Exampel 1 were tested. The retention of microspheres and the bulk/thickness of the paper were determined as in Example 1. The results are shown in Table 1 TABLE 2 VDC/ACN/MMA in Amount of Particle Increased bulk polymer shell propellant size (% per percentage of (wt %) (wt %) (μm) retained microspheres) 56/35/9 36.2 17.8 47 56/35/9 12.5 12.7 17 56/35/9 14.0 11.2 11 73/24/3 24.5 20.5 34 73/24/3 19.4 19.5 33 73/24/3 20.4 33.5 32 73/24/3 18.4 18.3 29 73/24/3 15.9 26.1 28 73/24/3 23.5 12.8 23 73/24/3 30.9 17.8 21 73/24/3 15.1 17.3 19 73/24/3 15.4 12.1 14 73/24/3 13.4 15.1 14 It appears that the overall trend is that the combination of high amount of propellant and large particle diameter gives high increase of the bulk of the paper. However, due to difficulties to exactly measure the amount of retained microspheres, some individual results may be inconsistent with the overall trend. 

1-17. (canceled)
 18. Process for the production of paper or nonwoven from fibres comprising the steps of adding thermally expandable microspheres comprising a thermoplastic polymer shell and a propellant entrapped therein to a stock comprising fibres or to a web of fibres, forming paper or nonwoven from the stock or the web, and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk of the paper or the nonwoven, wherein said expandable microspheres comprise from about 17 to about 40 wt % propellant and have a volume-average diameter from about 17 to about 35 μm according to ISO13319:2000.
 19. Process as claimed in claim 18, wherein the expandable microspheres have a volume-average diameter from about 19 to about 35 μm according to ISO13319:2000.
 20. Process as claimed in claim 18, wherein the expandable microspheres comprise from about 19 to about 40 wt % propellant.
 21. Process as claimed in claim 18, wherein the thermoplastic polymer shell is made of a co-polymer from ethylenically unsaturated monomers comprising at least one acrylic ester or methacrylic ester monomer and at least one vinylidene halide monomer.
 22. Process as claimed in claim 21, wherein the thermoplastic polymer shell is a co-polymer obtained from monomers comprising methyl methacrylate, vinylidene chloride and acrylo nitrile.
 23. Process as claimed in claim 18, wherein the propellant comprises isobutane.
 24. Process for the production of paper comprising the steps of adding expandable microspheres to a stock containing cellulosic fibres, dewatering the stock on a wire to obtain paper, and drying the paper by applying heat and thereby also raising the temperature of the microspheres sufficiently for them to expand and increase the bulk of the paper, wherein said expandable microspheres comprise from about 17 to about 40 wt % propellant and have a volume-average diameter from about 17 to about 35 μm according to ISO13319:2000.
 25. Process as claimed in claim 24, wherein paper comprising three or more layers is produced and the expandable microspheres are not added to the portion of the stock forming any of the two outer layers.
 26. Process as claimed in claim 24, wherein the cellulosic fibres partly or fully originate from recycled paper.
 27. Process as claimed in claim 24, wherein the expandable microspheres have a volume-average diameter from about 19 to about 35 μm according to ISO13319:2000, comprise from about 19 to about 40 wt % of isobutane as propellant, and the thermoplastic polymer shell is a co-polymer obtained from monomers comprising methyl methacrylate, vinylidene chloride and acrylo nitrile.
 28. Process for the production of nonwoven as claimed in claim 18, comprising the steps of forming a web of fibres, adding to said web a binder and expandable microspheres, and forming nonwoven and applying heat to raise the temperature of the microspheres sufficiently for them to expand and thereby increase the bulk nonwoven, wherein said expandable microspheres comprise from about 17 to about 40 wt % propellant, and have a volume-average diameter from about 17 to about 35 μm according to ISO13319:2000.
 29. Thermally expandable microspheres comprising a thermoplastic polymer shell made of a co-polymer obtained by polymerizing ethylenically unsaturated monomers comprising at least one acrylic ester or methacrylic ester monomer and at least one vinylidene halide monomer and from about 17 to about 40 wt % of a propellant entrapped in said polymer shell, wherein the expandable microspheres have a volume-average diameter from about 17 to about 35 μm according to ISO13319:2000.
 30. Thermally expandable microspheres as claimed in claim 29, wherein the ethylenically unsaturated monomers comprise acrylo nitrile.
 31. Thermally expandable microspheres as claimed in claim 29, having a volume-average diameter from about 19 to about 35 μm according to ISO13319:2000.
 32. Thermally expandable microspheres as claimed in claim 29, comprising from about 19 to about 40 wt % propellant.
 33. Thermally expandable microspheres as claimed in claim 29, wherein the blowing agent comprises isobutane.
 34. Thermally expandable microspheres as claimed in claim 29, wherein the expandable microspheres have a volume-average diameter from about 19 to about 35 μm according to ISO13319:2000, comprise from about 19 to about 40 wt % of isobutane as propellant, and the thermoplastic polymer shell is a co-polymer obtained from monomers comprising methyl methacrylate, vinylidene chloride and acrylo nitrile. 